n action potential is a rapid, all-or-none change in the membrane potential, followed by a return to the resting membrane potential. This chapter describes how action potentials are generated by voltage-dependent ion channels in the plasma membrane and propagated with the same shape and size along the length of an axon. The influences of axon geometry, ion channel distribution, and myelin are discussed and explained. The ways in which information is encoded by the frequency and pattern of action potentials in individual cells and in groups of nerve cells are also described. Finally, because the nervous system provides important information about the external world through specific sensory receptors, general principles of sensory transduction and coding are introduced. More detailed information about these sensory mechanisms and systems is provided in other chapters.
When a microelectrode (tip diameter <0.5 µm) is inserted through the plasma membrane of a neuron, a difference in potential is observed between the tip of the microelectrode inside the cell and an electrode placed outside the cell. The internal electrode is approximately 70 mV negative with regard to the external electrode, and this difference is referred to as the resting membrane potential or, simply, the resting potential (see for details on the basis of the resting potential). (By convention, membrane potentials are expressed as the intracellular potential minus the extracellular potential.) Neurons have a resting potential that typically is around −70 mV.
One of the signature features of neurons is their ability to change their membrane potential rapidly from rest in response to an appropriate stimulus. Two such classes of responses are action potentials and synaptic potentials, which are described in this chapter and the next, respectively. Current knowledge about the ionic mechanisms of action potentials comes from experiments with many species. One of the most studied is the squid because the large diameter (up to 0.5 mm) of the squid giant axon makes it an excellent model for electrophysiological research with intracellular electrodes.
The Passive Response
To understand how an action potential is generated and why it is needed, it is necessary to understand the passive electrical properties of the nerve cell membrane. The term passive properties refers to the fact that components of the cell membrane behave very similarly to some of the passive elements of electric circuits, including batteries, resistors, and capacitors. This is very useful because the properties of these elements are well understood. In particular, a piece of membrane containing ion channels responds to changes in voltage across it much as a circuit containing a resistor and capacitor in parallel (parallel RC circuit) would: The ion channels correspond to the resistor, and the lipid bilayer acts as a capacitor. When a battery is first connected across the two terminals of a parallel RC circuit, all of the current flows through the branch of the circuit with the capacitor, causing the voltage across it to begin changing (recall that for a capacitor, I ˜ dV/dt). Over time, however, the current flow through the capacitor decreases, whereas that through the resistor increases. As this happens, the rate of voltage change across the capacitor (and resistor) slows, and the voltage approaches a steady-state value. This change in voltage has an exponential time course whose specific characteristics depend on the resistance (R) and capacitance (C) of the resistor and capacitor. Moreover, a time constant, τ, for this circuit can be defined by the equation τ= R * C, and it equals the time it takes for the voltage to rise (or fall) exponentially by approximately 63% of the difference between its initial and final values.
With regard to how an axon actually responds to electrical stimulation,
Fig. 5.1 illustrates the results of an experiment in which the membrane potential of an axon is altered by passing rectangular pulses of depolarizing (upward-going 1.0 0.89 0.44 0.22 0.11 –0.11 –0.22 –0.44 –0.89 –1.0 0 5 10 15 20 25 Change in Voltage from baseline (mV)